1. Field of the Invention
The present invention relates to a video encoding method and a video decoding method, more particularly to a motion compensation predictive interframe encoding method and decoding method using a plurality of reference frames.
2. Description of the Related Art
In video encoding schemes such as MPEG 4 (International Organization for Standardization/IEC 14496-2) or MPEG4 Advanced Video Coding (H.264/International Organization for Standardization/IEC 14496-10), using a plurality of reference frames and a plurality of predictive modes, the video is encoded by selecting an optimum predictive mode and an optimum reference frame in units of a pixel block unit. This improves a predictive efficiency, and realizes high efficiency compression encoding. In such an encoding system, it is necessary for identifying a reference frame and a predictive mode to encode identification information identifying them and motion vector information (referred to as motion compensation predictive information) every encoding block.
For the purpose of improving the predictive efficiency more, if kinds of predictive mode are increased or information of a motion vector is encoded every block, the number of encoded bits of the motion compensation predictive information increases. As a result, improvement of the predictive efficiency and increase of the number of encoded bits of motion compensation predictive information are offset. In other words, the encoding efficiency is not improved in total.
To evade increase of the number of bits of motion compensation predictive information is used a methods for automatically generating motion compensation predictive information of a current block not by using motion compensation information of an existing block encoding but by using motion compensation predictive information in the peripheral pixel block which is already encoded or the other flame which is already encoded.
In MPEG-4 video encoding is adopted a scheme for generating motion compensation information from adjacent blocks or flames without encoding the motion compensation information in a direct mode and a predictive mode as referred to as a skipped macroblock. In generation of the motion compensation information, the correlation of motion compensation information is supposed in a frame or between frames every pixel block. Motion compensation parameters are estimated from peripheral motion compensation information according to a given rule. It is possible by using the same estimate rule in an encoding time and a decoding time to perform motion compensation prediction normally and decode the result.
In the video encoding standard scheme of ISO/IEC MPEG-4, a direct mode is used in bidirectional predictive interframe encoding (B picture) as an encoding mode. This encoding generates motion compensation predictive information using, as a predictive source, macroblocks of spatially the same position on an intraframe encoded frame (I picture) most near to a current frame with respect to time or an interframe predictive encoded flame (P picture). This method decreases redundancy of additional information such as motion vector information. Specifically, if a predictive source macroblock is an interframe predictive encoded frame (P picture), its motion vector is scaled according to a temporal position of a to-be-predicted macroblock to generate motion compensation predictive information.
It is a problem of this method that the to-be-predicted macroblock must be located between the predictive source macroblock and the reference frame of the predictive source macroblock. Therefore, when the to-be-predicted macroblock is predicted only by a plurality of past reference frames or when the predictive source macroblock has a plurality of reference frames, how to generate the motion compensation predictive information is not clear.
As described above, when the motion compensation information is estimated to decrease the encoding overhead of the motion compensation information, the encoding efficiency is controlled greatly according to the property of an estimation rule, that is, according to whether or not appropriate movement predictive information can be automatically generated. When the estimation of motion compensation information is not appropriate, the encoding efficiency deteriorates, resulting in degrading picture quality or increasing the number of encoded bits.
In addition, when a predictive picture is made only by a plurality of past reference frames or the predictive source has a plurality of reference frames, an appropriate direct mode cannot be realized.